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The growth in abuse of heroin is now considered the number one drug threat by the nation's police, overtaking methamphetamine, according to a new survey by the Drug Enforcement Administration.

"Heroin availability is up across the country, as are abuses, overdoses, and overdose deaths," says the 2015 National Drug Threat Assessment Summary, released Wednesday.

Law enforcement seizures of heroin have nearly doubled in the past five years, and the number of heroin users is up 51%, according to the latest findings from the Substance Abuse and Mental Health Services Administration.

The DEA report says deaths from drug overdoses climbed to 46,471 in 2013, the most recent year for which figures are available.

"Roughly half of the overdose deaths are related to abuse of prescription drugs and another 8,000 involve heroin. So combined those two things account for two-thirds of the overdose deaths," said DEA Administrator Chuck Rosenberg.

But he said another contributor may be what FBI Director James Comey has called "the Ferguson affect," with police officers more reluctant to leave their patrol cars, concerned they may become the subjects of the next viral video about violent police encounters.

"I've been hearing the same thing from police chiefs. I think Comey was spot-on," Rosenberg said in a wide ranging question and answer session with reporters at DEA headquarters in Washington. Rosenberg previously served as Comey’s chief of staff at the FBI.

"We're not entirely sure what's going on, and we ought to talk about it and try to figure it out. We need better data."

The DEA administrator also said he's concerned that the nation's cities may lack the capacity to deal with the number of federal prisoners being released under an Obama administration effort to free inmates who have served part of their sentences for non-violent drug offenses.

"If we suddenly de-institutionalize a whole bunch of people, do we have some other institutions to step up into the breach and give them the services and care they're going to need? Because, what happens if we don't? That's what worries me."

Pete Williams

Pete Williams is an NBC News correspondent who covers the Justice Department and the Supreme Court, based in Washington.